It is regularly necessary to remove gas components from a gas stream by adsorption on a solid adsorbent. In particular, pre-purifying steps are commonly used when performing cryogenic air separation processes. Thereby, periodic regeneration of the adsorbent is necessary as such removed gas components may be of intrinsic value or they may be contaminating gas components in the gas mixture.
In such processes the gas is conventionally fed in contact with a solid adsorbent contained in an adsorber vessel to adsorb the component or components to be removed and these gradually build-up in the adsorbent. The concentration of the removed component or components in the adsorbent gradually rises and if the process is continued for a sufficient period, the adsorbed components will break through the downstream end of the adsorbent bed. Before this occurs, it is necessary to regenerate the adsorbent.
For performing pre-purifying steps different processes such as Thermal Swing Adsorption (TSA), Pressure Swing Adsorption (PSA) and Thermally Enhanced Pressure Swing Adsoprtion (TEPSA) are applied.
In a PSA process the desorption is done by stopping the flow into the adsorbent of gas to be treated, depressurizing the adsorbent and, usually, by passing a flow of regenerating gas low in its content of the component adsorbed on the bed through the bed counter-current to the product feed direction.
The TSA process is commonly used to pre-purify air upstream of a cryogenic air separation unit (ASU). The TSA process is characterized by high temperature regeneration of the adsorption process, typically well above 100° C., and a long hot regeneration period. Since the heat flux associated with TSA processes is intense and lengthy, degradation in heat pulse quality has minimal impact on the TSA desorption process. The heat provided to a TSA process, by the heater, is primarily used to desorb the strongly adsorbed component, namely water, which has high heat of adsorption.
A variant of the TSA process is the TEPSA process described for example in U.S. Pat. No. 5,614,000. TEPSA is a low temperature regeneration process with temperatures typically below 100° C. and only with short heating. Contrary to the TSA process, the heat provided by a heater is used to remove the “least strongly” adsorbed contaminates, namely CO2. The heat flux associated with TEPSA processes can be described as weak, so even small heat losses on its journey towards the adsorbent bed can immensely degrade the quality of the heat carried by the heat pulse.
This is in contrast to the conventional TSA processes being dominated by high temperatures for regeneration of well above 100° C. and periods of hot regeneration of well above 10 minutes. Due to the more drastic conditions of TSA processes the position of the heater within reasonable distance (e.g. 20 m or more away from the adsorber vessel) has only little influence on the regeneration process, c.f. U.S. Pat. No. 9,108,145.
In U.S. Pat. No. 5,614,000 as well as U.S. Pat. No. 8,734,571 an apparatus configuration for TEPSA is described including only a single external heater for providing hot regeneration gas. Such a configuration, only including one external heater not being located in close proximity to the adsorbent vessels, has the disadvantage that heat losses occur easily, thereby degrading the quality of the heat pulse (which ideally has rectangular shape), sent to the adsorbent bed immensely.
This is the reason why in the practise of TEPSA processes so far such a configuration has not been applied, but rather configurations such as described in U.S. Pat. No. 7,066,986 are used. In this document, a two bed heater arrangement for TEPSA processes is disclosed wherein every adsorbent vessel has a separate heater. The heaters are arranged in a way that every adsorbent vessel is equipped with a separate heater element being located in the inlet nozzle of the adsorber vessel. By such a TEPSA arrangement described above disadvantages such as heat loss, varying quality of the heat pulse etc. can be minimized. Besides, the distance between the heater and the adsorbent bed is minimised, maintaining the quality of the heat pulse prior to contact with the adsorbent bed.
The problem with a heater arrangement as described by U.S. Pat. No. 7,066,986 is that multiple heaters have to be used, that is, one for each adsorbent vessel. Such an arrangement causes increased maintenance efforts due to the fact that a multitude of heaters is necessary and that these heaters have to be positioned in close contact with the adsorbent vessel. Furthermore, due to the fact that the heaters are contained in the inlet nozzles of the adsorbent vessels they hinder the implementation of more complicated vessel arrangements due to their bulkiness. In addition, a multitude of heaters has cost adverse effects due to an increase in material and energetic input. Furthermore, maintenance costs are also higher.
Thus, there is the need of an improved TEPSA process. The present invention aims to overcome the disadvantages of the TEPSA processes known in the art, and particularly aims to provide a process requiring a less complicated and expensive apparatus configuration which concurrently provides a stable heat pulse to the adsorbent bed(s).
Thus, the present invention aims to process intensification for operating low regeneration temperature TEPSA, related to the purification of air prior to cryogenic separation of air, to simplify and reduce the cost of the current process.
Moreover, the present invention relates to the provision of an apparatus which can be used for such TEPSA processes.